At 10:45 p.m. ET on Sunday, a report posted on Yahoo's Mail Status page reported that company engineers had been working on the email problems over the weekend and were "making steady progress" on restoring lost messages. The Yahoo team is restoring folders and the tools that can tell users whether a message has been read.

"As part of the restoration process, some timestamps may not appear correctly on some of your messages," the company said in the post. "Thank you again for your continued patience through this process and we will share an update again tomorrow."

The Mail Status page has not yet been updated.

Patrick Moorhead, an analyst at Moor Insights & Strategy, described the email system crash and continued problems a huge blow to Yahoo, which has been trying to overhaul its image as an Internet has-been.

"I don't recall any other service like this being down this long over the past 15 years," he added. "It makes Yahoo look like a real Web rookie. If Yahoo doesn't fix [it soon], day-by-day they will continue to lose more credibility as a Web services company."

The gains made by CEO Marissa Mayer over the past year are now in jeopardy, Moorhead noted.

"Mayer has built some goodwill for Yahoo since her tenure, slowly shifting the perception from a has-been to a company that has a chance," said Moorhead. "This incident will surely hurt any perception of improvement. People must be thinking, 'Email is so '90s. How can they do mobile or search well?' "

"For many of us, Yahoo Mail is a lifeline to our friends, family members and customers," Mayer wrote in a Tumblr post. "This week, we experienced a major outage that not only interrupted that connection, but caused many of you a massive inconvenience. That's unacceptable and it's something we're taking very seriously. Unfortunately, the outage was much more complex than it seemed at first."

She added that users are affected differently, making the restoration process all the more complex.

"Above all else, we're going to be working hard on improvements to prevent issues like this in the future," Mayer wrote. "While our overall uptime is well above 99.9%, even accounting for this incident, we really let you down this week. We can, and we will, do better in the future."

Yahoo's mail service began floundering around 1:30 a.m. ET on Tuesday, Dec. 10, when users began reporting that they couldn't access email in their inboxes, and that emails they had sent weren't being delivered.

Yahoo had told users a few times last week that the service would be back up and running correctly within a few hours.